1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly for optimization of dielectric circuit board materials for improved performance in four port circuits such as four port branch line couplers.
2. Description of the Related Art
RF circuits such as quarter-wave transformers and four port branch line couplers are commonly manufactured on specially designed substrate boards. For the purposes of RF circuits, it is important to maintain careful control over impedance characteristics and electrical length. If the impedances of different parts of the circuit do not match, this mismatch can result in inefficient power transfer, unnecessary heating of components, and other problems. A specific type of transmission line structure often used to match the impedances of different parts of the circuit is a quarter-wave transformer. Hence, the performance of quarter-wave transformers in printed circuits can be a critical design factor.
As the name implies, a quarter-wave transformer typically has an electrical line length of xcex/4, where xcex is the signal wavelength in the circuit. As is well known by those skilled in the art, the proper characteristic impedance of a quarter-wave transformer is given by the formula Z0={square root over (Z1Z2)}, where Z0 is the desired characteristic impedance of the quarter-wave transformer, Z1 is the impedance of a first transmission line to be matched, and Z2 is the impedance of a second transmission line or load being matched to the first transmission line.
Printed quarter-wave transformers used in RF circuits can be formed in many different ways. Three common implementations are described below. One configuration known as microstrip, places the quarter-wave transformer on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the quarter-wave transformer is covered with a dielectric superstrate material. In a third configuration known as stripline, the quarter-wave transformer is sandwiched within substrate between two electrically conductive (ground) planes.
Two critical factors affecting the performance of a substrate material are permittivity (sometimes called the relative permittivity or ∈r) and the loss tangent (sometimes referred to as the dissipation factor). Another critical factor is the permeability (sometimes called the relative permeability or xcexcr). The relative permittivity and relative permeability determine the speed of the signal, and therefore the electrical length of transmission lines and other components implemented on the substrate. The loss tangent characterizes the amount of loss that occurs for signals traversing the substrate material. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Ignoring loss, the characteristic impedance of a standard transmission line, such as stripline or microstrip, is equal to {square root over (L1/C1)} where L1 is the inductance per unit length and C1 is the capacitance per unit length. The values of L1 and C1 are generally determined by the physical geometry and spacing of the line structure as well as the permittivity and permeability of the dielectric material(s) used to separate the transmission line structures. Conventional substrate materials typically have a relative permeability of approximately 1.0.
In conventional RF design, a substrate material is selected that has a relative permittivity value suitable for the design. Once the substrate material is selected, the line characteristic impedance value is exclusively adjusted by controlling the line geometry and physical structure.
The permittivity of the chosen substrate material for a transmission line, passive RF device, or radiating element influences the physical wavelength of RF energy at a given frequency for that line structure. One problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is optimized for all of the various passive components and transmission line circuits to be formed on the board. In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide respectively for practical implementation for a given substrate. Since the physical size of the microstrip or stripline is inversely related to the relative permittivity of the dielectric material, the dimensions of a transmission line can be affected greatly by the choice of substrate board material.
Still, an optimal board substrate material design choice for some components may be inconsistent with the optimal board substrate material for other components, such as antenna elements. Moreover, some design objectives for a circuit component may be inconsistent with one another. Accordingly, the constraints of a circuit board substrate having selected relative substrate properties often results in design compromises that can negatively affect the electrical performance and/or physical characteristics of the overall circuit.
An inherent problem with the foregoing approach is that, at least with respect to the substrate material, the only control variable for line impedance is the relative permittivity, ∈r. Changes in the relative permittivity affect C1, the capacitance per unit length. This limitation highlights an important problem with conventional substrate materials, i.e. they fail to take advantage of the other material factor that determines characteristic impedance, namely the relative permability, xcexcr. Changes in the relative permeability affect L1, the inductance per unit length of the transmission line.
Yet another problem that is encountered in RF circuit design is the optimization of circuit components for operation on different RF frequency bands. Line impedances and lengths that are optimized for a first RF frequency band may provide inferior performance when used for other bands, either due to impedance variations and/or variations in electrical length. Such limitations can reduce the effective operational frequency range for a given RF system.
Conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the permittivity. Accordingly, conventional dielectric substrate arrangements for RF circuits have proven to be a limitation in designing circuits that are optimal in regards to both electrical and physical size characteristics.
The present invention relates to a circuit for processing radio frequency signals. The circuit includes a substrate where the circuit can be placed. The substrate can include a meta-material (which are described in more detail later) and can incorporate at least one dielectric layer. A four port circuit and at least one ground can be coupled to the substrate.
The dielectric layer can include a first region with a first set of substrate properties and at least a second region with a second set of substrate properties. The substrate properties can include permittivity and permeability. The second set of substrate properties can be different than the first set of substrate properties. In one embodiment the permittivity and/or permeability of the second region can be higher than the permittivity and/or permeability of the first region. Further, the first and/or second set of substrate properties can be differentially modified to vary a dielectric permittivity or magnetic permeability or both over a selected region. The dielectric layer can further include other regions with different sets of substrate properties.
At least a portion of the four port circuit can be coupled to the second region. The increased dielectric permittivities or permeabilities or both can reduce a size of the four port circuit. The increased permittivities or permeabilities or both also can effect a change in at least one of a impedance, an inductance, a capacitance, a quality factor (Q) and a voltage associated with the four port circuit.